Spinal support plates

ABSTRACT

A spinal plate is provided that has two components of curvature in the lower surface that opposes the spinal vertebrae. This lower surface preferrably has the shape of a toroidal segment. A series of plates may be taken from a principal &#34;s&#34;-shaped template of similar shape on its lower surface. For convenience of manufacture, the shape of the curvature of the &#34;S&#34;-shaped template along the sides of the template is circular. A special hole pattern is also disclosed in respect of a tapered thoracic portion of the template.

FIELD OF THE INVENTION

This invention relates to support and reinforcement plates for portionsof the human spine. More particularly, it relates to a new shape forsuch plates, and a template for providing a series of spinal supportplates that are dimensioned to be applied to specific regions of thehuman spine, for a broad range of patients.

BACKGROUND TO THE INVENTION

Surgical treatment of a degenerative spinal condition has for some timerelied upon the attachment to consecutive spinal vertebrae ofstabilizing, reinforcing plates. Such bone plates have been attached tothe vertebrae by means of screws that are set into the bone. Thesurfaces of such plates bearing against the bone have been both flat andcurved about a single axis. Attempts have been made to introducespecific configurations for such plates in order that they may fit moreintimately against the bones they are engaging.

For background, reference may be made to an article in CLINICALORTHOPAEDICS AND RELATED RESEARCH Number 227, February 1988, page 135entitled: "A Contoured Anterior Spinal Fixation Plate".

In terms of geometry, a spinal plate should be as wide as possible incross-section in order to span the lateral side of the vertebrae towhich it is attached. It should not protrude so as to interfere withblood vessels or nerve tissue. And preferably, in its long axis itshould substantially follow the natural contour of the spine so that thevertebrae being reinforced may be held approximately in their normalorientation to each other.

It has been customary to produce spinal plates that are curved about asingle longitudinal axis. Examples include U.S. Pat. Nos. 1,105,105;3,695,259; 4,454,876; 4,493,317; and 4,683,878 and the plate describedin the above Clinical Orthopaedics article. This curvature tends tocreate two parallel line-contacts between the plate and the vertebrae towhich it is fastened so long as the cross-sectional radius of the plateis less than that of the vertebral body. Subject to irregularities inthe vertebrae, such line contacts, when they occur at the outside edgesof the spinal plate, create a maximum span between the opposed points ofengagement with the vertebrae. This tends to improve and maximize thestability of the coupling between the plate and each vertebra.

While spinal plates in the past have been bent about a single axis ofcurvature, no successful attempts have been made, prior to thisinvention, to produce spinal plates that have two simultaneous forms orcomponents of curvature.

Care must be taken in selecting the geometry of such plates to obtain anoptimal compromise between features of shape and cost of manufacture.This is particularly true in the case of spinal fixation plates where avariety of different plate shapes are to be preferred, in accordance tothe specific vertebrae to which they are to be attached.

It might generally be thought that the individual variety in humanspinal geometry would optimally require that plates for fixation tospecific individuals, should be custom shaped for that individual. Thisis not practical since no convenient means presently exists for thepre-surgical extraction of precise spinal dimensions; and spinal plates,at least those made of hard metal, such as plate steel, cannot beconveniently re-shaped during surgery.

While an ideal fit for each individual is not presently obtainable, itwould be desirable to identify a preferred geometry for spinal platesthat will allow such plates to be mass produced for large numbers ofindividuals, and still be reasonably close to the optimal geometry foreach individual.

The challenge of defining a standard geometry for spinal plates isfurther complicated by the fact that spinal plates, particularly for thethoracic region, should preferably be manufactured with two separatecurvatures embodied therein. As well, for the thoracic region, suchplates should also be tapered in their width. In order to facilitatemanufacture, it is desirable to reduce the geometry of such plates to aminimum of criteria that may readily be converted into manufacturingoperations that can be carried-out by existing production machinery.Computer-controlled machine tools, operating on the basis of suchcriteria, may then be used to mass manufacture such plates.

It is accordingly an object of this invention to provide for a form ofspinal plate that has a lower spine-opposing face that incorporates twodistinct patterns or components of curvature.

It is further an object of this invention to define a template fromwhich thoracic and lumbar spinal fixation plates may be cut which willfit, it is believed, a substantial proportion of human patients wheresuch plates are required.

It is a still further object of this invention to provide a criteria forthe distribution of screw holes on spinal plates that is optimal for theapplication for which such plates are intended.

The invention in its general form will first be described, and then itsimplementation in terms of specific embodiments will be detailed withreference to the drawings following hereafter. These embodiments areintended as examples to demonstrate the principle of the invention, andthe manner of its implementation. The invention implicit in suchembodiments, will then be further described, and defined, in each of theindividual claims which conclude this Specification.

SUMMARY OF THE INVENTION

According to the invention, a preformed spinal plate is provided thatcomprises a spine-opposing face incorporating two patterns or componentsof curvature. More particularly, a spinal plate having a median line isprovided with a lower surface that is shaped to lie along the uppersurface of a torus, with its outer longitudinal edges lyingsubstantially symmetrically astride the upper circumferential medianline of the torus whereby such longitudinal edges are located atsubstantially an equal distance from the upper circumferential medianline of the torus. The upper, circumferential median line of a torus, asdefined herein, is the line of contact between a torus and an overlyingplane.

The invention also comprises any one of a series of spinal reinforcementplates taken from a portion of a principal template that definessatisfactory shapes for lateral plates that may be applied to the 10lower thoracic vertebrae from T-3 to T-12; and the five lumbar vertebraeof the human spine, inclusive.

The invention also comprises a pattern of screw holes formed in a platefor use in the thoracic region of the spine, such plate having aspine-opposing face incorporating two components of curvature, andnarrowing or tapering sides.

The principal template is characterized by a reflexive "s"-shapedcurvature in its longitudinal axis, corresponding to the general shapeof the human spine in profile. This curvature is preferably slightlyless than that of the human spine in order that such plates may be usedto slightly straighten the spine, once installed.

The template may be chosen to fit the human spine or either the left orright side. Accordingly, ensuring descriptions of the template andspinal plates are intended to govern the form of the mirror image to theshapes being described.

The median line of the principal template, being oriented in generalalignment with of the overall longitudinal "axis" of the principaltemplate, lies substantially above the upper, circumferential medianline of a series of imaginary toroidal surfaces.

The principal template is geometrically divided into three portions. Afirst, lower curved portion of the principal template, corresponding tothe lumbar region of the human spine, is formed with inner and outersides lying along portions of two near-parallel arcs having first(outermost) and second (inner) radii of approximately equal lengths.These radii are each preferably of 13.50 and 12.71 inches in lengthrespectively and extend respectively from a pair of first and secondcenters, the coordinates for the first center being 5.44;-12.1 and forthe second center being (5.42;-12.31). (The Origin for such co-ordinatesbeing defined subsequently, below).

The differences in the lengths of the radii and the locations of theircenters are intended to provide for smooth transitions betweenrespective portions of the principal template, as further describedbelow.

The lowermost terminal end of the template, beyond the commencement ofthe first, lower curved portion, is preferably tapered at about 45degrees to a rounded point, conveniently of about 0.25 inches radius.This tapered portion protects the femoral arteries from abrasion onplate corners where these vessels branch-out from the base of the spine.

The first, lower curved portion of the principal template is defined ascommencing at its lower end where it joins with the above describedtapered portion, and extends to a first juncture with the next portionof the template. This first juncture corresponds in location with thethoracolumbar junction of the human spine. The length of this firstportion may be defined by reference to the median line that passes downits length. The span of the arc which approximates the median line forthe lower portion is preferably of about 20 degrees.

The origin for the system of coordinates which follows is the edge ofthe template at the thoracolumbar junction, on the side of the centersfor the first and second radii.

The median line for the entire principal template is approximated ineach portion as the arc which passes through three point positioned atthe midpoints of each portion (measured widthwise), such points beinglocated at the two ends and at the longitudinal center point of eachportion, e.g. half-way between the two ends. From the geometry alreadyprovided, the co-ordinates for the center and the radius of the arc thatapproximates the median line for the first portion of the principaltemplate are: (5.41; -12.69) and r=13.58 respectively.

Such arc will hereafter be referred to as being the "median line" in thefirst, lower curved portion of the principal template.

A second, central curved portion of the principal template,corresponding to the lower region of the thoracic vertebrae above thethoracolumbar junction and commencing at the first juncture andextending to a second juncture, is formed with inner and outer sideslying along portions of two near-parallel inner and outer arcs havingthird and fourth radii. These radii are respectively preferably of 28.32and 29.88 inches in length and have third and fourth centersrespectively which lie on the opposite side of said template from thefirst and second centers.

The coordinates of the third center are (-0.29; 29.32). The co-ordinatesof the fourth center are (-0.41; 29.87). Thus, this third displacementcombined with the third and fourth radii determine the width of theplate in the second central portion of the template.

The point where the arc of the fourth radius (defining the outer side ofthe second portion of the principal template) meets with the secondjuncture is the origin for this description, in cartesian co-ordinates.

The angular span of the second central portion of the principal templateis 8 degrees, from the first juncture to the second juncture where thenext portion of the principal template begins as measured along themedian line is preferably 8 degrees. The center point for the medianline in this central portion has as co-ordinates (-0.53; 29.70) and theradius defining the median line is 29.21 inches.

A third, upper thoracic portion of the principal template, commencing atthe second juncture, is formed with inner and outer sides lying alongportions of two intersecting arcs having fifth and sixth radii. Thelengths of the fifth and sixth radii, and the locations of the fifth andsixth centers from which they respectively extend, cause the sides ofthe principle template to progressively narrow, proceeding from thesecond juncture towards the upper end of the principal template. Thecorners at the upper terminal end of the template are preferably roundedto avoid the presence of damaging, protruding corners.

These fifth and sixth radii are respectively preferably 28.63 inches and24.46 inches in length. The co-ordinates of the fifth center are (-0.78;29.62).

The co-ordinates of the sixth center are (-9.01; 24.26).

The angular span of the third upper portion of the template along itsmedian line is preferably about 17 degrees from the second juncture tothe upper, terminal end of the principal template. The center point forthe median line in this portion has co-ordinates of (-0.36; 26.82) andthe arc for the median line has a radius of 26.33 inches.

Through use of the geometry employed, the principal template is providedwith sides of precisely circular curvature within each of the threeportions. Further a line approximating the median line in each portionhas been defined which is also circular in curvature within eachportion. While actual dimensions have been provided, these precisevalues may be varied. The principal being demonstrated is that atemplate for spinal plates may be defined using the basic geometry thathas been described. The rationale for this geometry will now beelaborated.

The width of the first portion of the principal template is determinedby the first displacement between the first and second centers and therespective lengths of the first and second radii. This width, ispreferably one inch, but not exactly so. By reason of the geometricconstruction employed, the width varies by a few thousandths of an inchalong its length as explained further below.

The width of the second portion of the principal template is determinedby the third displacement between the third and fourth centers and thelengths of the third and fourth radii. The width of the second portion,is also preferably one inch but will vary as in the first portion.

The width of the third portion of the principal template is determinedby the taper of the opposed sides, as proceeding from the secondjuncture to the upper end of the principal template. The preferred widthof the upper end, suitable for plates that extend to the tenth thoracicvertebrae is 0.63 inches, based on a width of one inch at the secondjunction.

In the lumbar and central portions, it has been indicated that, whilethe template is substantially of a constant width, this width is notexactly constant. This feature arises because, for convenience ofmanufacture, the edges are preferably defined by circular curves. Thesetwo criteria of circular edges and constant width would be met exactlyonly if the respective sides are based on circular arcs that have thesame center.

In such a case, the inside arc must have a smaller radius, and thereforea higher curvature.

The curve of the template reflexes at the thoracolumbar junction. Thecentral portion of the template is also of substantially constant widthwith preferably circular boundaries. Where these two curved portions ofthe principal template meet, it is desirable to minimize the degree ofdiscontinuity that occurs at the intersection. This would be bestachieved by providing that the tangents to the respective radii bealigned or co-linear at the point of intersection.

At the point of intersection of the first and second portions of thetemplate, i.e. at the first juncture, the inner radius of the lumbarregion intersects with the outer radius of the central portion, and theouter radius of the lumbar region intersects with the inner radius ofthe central portion. If the sides in each respective portion were tohave a common center then the inner radius in one portion, havinggreater curvature, would intersect with an outer radius lo in the otherportion, of lesser curvature. Thus two curves of opposed and differingcurvature would be meeting. It is considered desirable to minimize thedifferences in the degrees of curvature at such junctions in order tomake the transitions between the sides at the junction more smooth andregular, e.g. more nearly symmetrical.

Higher symmetry at the junction could be achieved if the reflexing arcportions each possessed the same radii of curvature. However, an objectof the design of the template is to produce spinal plates for the lumbarand central thoracic portions of the spine that are of substantiallyconstant width. This criteria of constant width cannot be achievedexactly where both the inner and outer radii in each portion of thetemplate are of equal lengths.

A compromise may, however, be adopted by choosing an inner radius thatis shorter than the outer radius for each portion by an amount which isless than the width of the template; and by also displacing the centerfor the inner radius by a distance equal to the complementary lengthnecessary to provide a template portion of the desired constant width atits end portions. Such a construction will provide a template of nearlythe same width in between, with the variances in width being only on theorder of a few thousandths of an inch.

Thus, by this means a criteria is provided by which the inner radii inthe two intersecting template portions are "flattened out" somewhat toprovide smoothness of transition at the juncture between the twoportions. At the same time, the width of the template in each portion ismaintained at a constant value, not only at the ends of each portion,but also at the midpoint. While not precisely exact, this provides atemplate of substantially constant width passing from the lower end ofthe lumbar portion up to the upper end of the central portion.

It is for this reason that the preferred embodiment as described hasdiffering centers and radii for each of the defining boundary portionsof the principal template.

The foregoing description defines a principal template having threeportions, as viewed from a longitudinal plan view. The shape of theprincipal template is cross-sectional view will now be described.

The principal template is characterized in its transverse shape alongits longitudinal axis by a lower spine-opposing surface shape that,within each of its portions, is defined by or shaped to lie along theupper surface of a toroidal segment. A first toroidal segment, centeredat a point directly below the center of the arc that approximates themedian line of the principal template in the first portion, defines thelower surface shape of the template in the first, lower lumbar portion.Second and third toroidal segments, centered on the opposite side of theprincipal template define the lower surface shape of the template insuch second and third portions.

All of the toroidal segments lie with their principal radii (meaning theradii from the center of the torus to the center point of each circledefined by radial cross-sections taken through the torus) located in thesame plane. Further the upper, circumferential median lines of eachtoroidal segment (being the line of contact between the torus and anoverlying plane) intersect their adjacent segment at the junctures ofthe principal template. All toroidal segments have a common tubulardiameter, preferably of two inches, and have surfaces that intersect ina relatively smooth, continuous manner, particularly along their uppercircumferential median lines.

Each of the toroidal segments have a center and a principal diameterthat causes the upper circumferential median line of the torus toconform as closely as possible to the median line of the principaltemplate, as projected onto its lower surface. Thus the respectivetoroidal segments are centered at points that are displaced directlydownwardly below the respective centers for the arcs approximating themedian lines in the various portions of the principal template. Theamount of downward displacement is equal to the minor radius of eachtorus and the principal radius for each toroidal segment corresponds tothe radius for each of arcs defining portions of the median line.

While use of three toroidal segments is preferred, the second segmentutilized for the central portion of the template may optionally beformed as a no toroidal extension of the third toroidal segment formaking plates that do not extend over the first juncture.

Where only a minor portion, e.g., equivalent to one vertebrae, of aplate taken from the upper or lower portions of the principal templatelies across one of the junctures, it is permissible to substitute asimpler shape for the toroidal segment underlying the central portion ofthe principal template. This substituted shape may be a straightcylindrical extension to either of the toroidal segments utilized forthe upper or lower portions of the principal template.

This substitution is permissible only for minor intrusions across thefirst and second junctures. Otherwise the preferred shape for the torusunderlying the principal template should be followed.

Individual plates may be patterned on portions of the principal templateconstituting sub-templates. Starting from the lower end of the principaltemplate, and proceeding along the median line of the template,sub-templates may be selected that are of a sufficient length to extendbetween the vertebrae to be supported. Preferably this should be limitedto between two and six vertebrae, inclusive.

The lowermost sub-template for the bottom-most lumbar vertebrae shouldbe tapered at its lower end, as described above, to minimize the risk ofinterference with the femoral arteries which branch in this region.Otherwise, sub-templates may have square-cut ends with rounded cornersso long as the hole pattern, as defined below, permits holes to lie inopposed pairs, on opposite sides of the median line. When the holepattern is staggered, as where sub-templates are taken from thenarrowing, upper thoracic region, then the ends of the sub-templates arepreferably tapered, following the separation limits for theclose-packing rules for the holes, as described below.

To attach the various plates selected from sub-15 templates to thespinal vertebrae, any conventional pattern of screw-holes may be formedin each sub-template. A preferred pattern of screw holes for asub-template will now be described. This preferred pattern of screwholes is based on spherically counter-sunk holes of the known type, allhaving a standard, fixed outside diameter over the entire length of thesub-template.

Within the first and second portions the holes are distributed intriplets down the length of the sub-template. The central hole of eachtriplet is centered on the median line of the principal template, solong as the remaining two holes in each triplet may be placed inaccordance with the next following criteria. The remaining two holes ineach triplet are located with their centers symmetrically disposed onopposed sides of the median line, each as closely placed to the centralhole in its own and adjacent triplet as possible, while remainingseparated from any adjacent hole by a center-to-center distance of atleast equal to three times the radius of the counter-sunk portion ofeach hole. Holes are also spaced from the sides of the principaltemplate by a center-to-edge distance equal to at least twice the radiusof the counter-sunk portion of each hole. Subject to the aboverequirements, the holes in triplet sets are as closely packed aspossible.

Eventually, as the template narrows, these criteria cannot be met. Thisoccurs in the preferred embodiment at the position corresponding to thejuncture between the vertebrae T-10 and T-9. Thereafter, the holes arelocated in staggered pairs, one on each side of the median line of theprincipal template, spaced according to the closely packed and minimumseparation criteria set-out above. That is to say the holes areseparated from adjacent holes by at least three radii and from thetemplate boundary by at least two radii.

The preferred screw hole pattern may be positioned to commence at thelower end of the template, with the closest screw-holes separated fromsuch end by a distance equal to twice the radius of the counter-sunkportion of each hole. In successive sub-templates, the hole positionsmay be shifted from that in the principal template to ensure that theclosest screw holes to the end of each sub-template are spaced from suchend by the same distance as above.

Holes along the median line are drilled vertically through the principaltemplate. Holes along either side of the median line are drilled alonglines inclined away from the vertical and angled generally towards thecenter of the tubular portion of the toroidal segments, preferably at anangle of seven degrees from the vertical in the first and secondportions of the principal template.

A preferred material for plates made from the principal template is 316LVM surgical steel. With such a material a preferred thickness is 0.187inches. Such plates are not intended to be load bearing, but will serveto stabilize vertebrae while a new load-bearing element, e.g.: bonegraft or bone cement, is stabilizing in place.

The curvature of the lower surface of the template in the lower andcentral portions of the template is selected to be greater than that ofthe typical vertebrae against which the spinal plates patterned thereonare intended to lie. This provides two-spaced lines of contact betweenthe plate and the bone of each vertebrae, and minimizes the chances of a"rocking" contact being formed. It also causes the plate to engage thevertebrae with the widest possible "stance". A radius of curvature ofone inch throughout the entire principal template has been foundsatisfactory based on the examination of cadavers. On occasion anindividual may have an irregularity in the shape of their spinalvertebrae that prevents the plate from seating itself on its outsideedges. In such cases, the protruding portion of the bone should beshaved off.

In the upper portion of the template where the sides are distinctlytapered, the span between the sides of the curved lower surface of thetemplate is narrowed. As the sides of the vertebrae bodies in thethoracic region of the spine are less curved than in the lumbar region,this decrease in span ensures that plates taken from sub-templates inthis region will lie in satisfactory proximity to the bone to which theyare attached.

The template as described may be used to produce plates fromsub-templates for application to either the right or left side of thehuman spine. For the lumbar region it has been found practical to applyplates from the left side. For the thoracic region it is preferable thatsuch plates be applied from the right side.

The foregoing summarizes the principal features of the invention. Theinvention may be further understood by the description of the preferredembodiments of the invention in conjunction with the drawings, which nowfollow.

In summarizing the invention above, and in describing the preferredembodiments below, specific terminology has been resorted to for thesake of clarity. However, the invention is not intended to be limited tothe specific terms so selected, and it is to be understood that eachspecific term includes all technical equivalents which operate in asimilar manner to accomplish a similar purpose.

SUMMARY OF THE FIGURES

FIG. 1 is a perspective view of a human spinal column indicating therange of vertebrae over which the invention is to apply.

FIG. 2 is a schematic of spinal vertebrae from L-5 to T-3.

FIG. 3 is a plan view of the principal template of the invention, withscrew-holes in place, aligned with FIGS. 2 and 1.

FIG. 4 is a plan view of the principal template without screw-holes inplace, showing the centers for each of the principal radii.

FIG. 4a is an enlarged view of the lumbar portion of FIG. 4.

FIG. 4b is an enlarged view of the central portion of FIG. 4.

FIG. 4c is an enlarged view of the thoracic portion of FIG. 4.

FIG. 5 is a perspective view of two intersecting toroidal segments thatdefine a portion of the shape of the lower surface of the spinaltemplate.

FIG. 6 is a cross-sectional view of a spinal plate shown positioned onan anvil shaped in the form of one of the toroidal segments.

FIG. 7 is a plan view of a spinal plate taken from a sub-template forthe lumbar region showing the criteria for the preferred placement ofscrew-holes.

FIG. 7a is a plan view of a spinal plate taken from a sub-template forthe thoracic region where the principal template narrows, showing thecriteria for the preferred placement of screw-holes.

FIG. 8 is a cross-sectional view through the plate of FIG. 7 showing thealignment of the screw holes with screws engaged to a vertebrae.

FIG. 9 is a plan view of the principal template showing a pattern ofscrew holes, and selected examples of sub-templates extracted therefrom.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 a spinal column 1 is shown as having vertebrae 2 in the lumbarregion 3 and in the thoracic region 5. The thoracolumbar junction 4 isalso identified. FIG. 2 is a schematic to indicate the numbering of thevertebrae. FIG. 3 is a plan view of the principal template 7 made inaccordance with the invention, aligned with the vertebrae 2 againstwhich it is dimensioned to fit. The principal template 7 is intended toprovide a series of sub-templates for producing spinal plates 50A foruse between the fifth lumbar vertebrae and third thoracic vertebrae.

In FIG. 4 the outline of the principal template 7 is shown in plan view.A first, lower, curved portion 8, shown as an enlargement in FIG. 4a,commences at the lower end 9 and extends to the first juncture 10 whichforms the upper end 10 to the first portion 8. A second central portion11, shown as an enlargement in FIG. 4b, extends to the second juncture12. A third upper portion 13, shown as an enlargement in FIG. 4c,extends to the upper end 14.

In the first portion 8 first outer 16 and inner 15 sides are defined byarcs of first outer 18 and second inner 17 radii located at first 19 andsecond 20 centers on one side of the principal template. The width 94 ofthe template 7 is determined by the displacement between these two arcswhich define the inner 15 and outer 16 sides. The length of the first,lower portion 8 is identified by the 20 degree span between the firstportion bounding radius 21 extending to the median line 37 from theother first portion median center point 39 at the first juncture 10 andthe first portion bounding radius 21a extending to the lower end 9.

Adopting an arbitrary origin for a cartesian system, the first center 19is located at (5.44; -12.10) and the second center 20 is located at(5.42; -12.31). The first radius 17 is 13.50 inches in length and thesecond radius 18 is 12.71 inches long.

As can be seen from FIG. 4, beyond the lower end 9, the principaltemplate 7 is formed with tapered sides 95 angled at about 45 degreeswhich taper to a rounded point 22 preferably having a radius of 0.25inches.

The second central portion 11 of the principal template 7 has third,inner 23 and fourth outer 24 radii centered at third 25 and fourth 26centers with coordinates of (-0.29; 29.32) and (-0.41; 29.87)respectively. These define the second portion inner 90 and outer 91sides. The median line 37 of the principal template 7 is continuousthrough the first 10 and second 12 junctures into the third, upperportion 13 of the principal template 7. The angular span between the twosecond portion bounding radii 23a, 23b extending from the second portionmedian center point 38 for the median line in the second portion to thefirst junction 10, and the second juncture 12, constituting the ends ofthe second portion, is 8 degrees.

The inner 29 and outer 28 sides of the third upper portion 13 of theprincipal template 7 are defined by arcs respectively centered at afifth center 33, having as coordinates (-0.78; 29.62) and at a sixthcenter 34 with coordinates (-0.01; 24.26). The third portion inner 30and outer 31 radii defining the outer and inner boundaries 28, 29 arerespectively 24.46 inches and 28.63 inches in length.

The angular span of the third upper portion 13, along its median line 37is 17 degrees. The center 32 for this curve has coordinates of (-0.36;26.82) and a third portion bounding radius 32a extending to the upperend 14 of the median line 37 from the second juncture of 26.33 inches.

In FIG. 5 a schematic depiction of the intersection of two reflexivelycurved, intersecting toroidal segments 80, 81 is provided. The torusesfrom which they are derived have substantially the same major or minordiameters, and their principal radii lie with the same plane. A firsttoroidal surface segment 35 may be seen as analogous to the torus thatdefines the lower surface 36 of plates 40 formed from the principaltemplate 7 in its lower portion 8. A second toroidal surface segment 92may be seen as analogous to the torus that defines the lower surface 36of plates 40 formed from the principal template 7 in its central portion11. The toroidal surfaces 35, 92 preferably have identical tubulardiameters of 2 inches and are continuous at their juncture 48. An uppercircumferential median line 49 of the toroidal segments 80, 81 may beseen to run smoothly and continuously across the surfaces 35, 92 of bothtoroidal segments 80, 81. It is along this circumferential median line49 that the median line 46 of a plate 40, corresponding to the medianline 37 of the principal template 7, is aligned when forming the shapeof the lower surface 36 of plates 40 taken from the principal template7.

This forming process is shown in FIG. 6 where a sample plate 40 having alower surface 36 conforming to the lower surface of the principaltemplate 7 (taken, for example, from the first portion 8) is placed on atoroidally shaped anvil 41. An upper stamping head 42 mounted in a press(not shown) has a complementary, curved forming surface 43 correspondingto that of the upper surface 44 of the anvil 41. The non-existentcontinuation or extension of the toroidal shape of the anvil 41 is shownin ghost outline 45.

In pressing the plate 40, the median line 46 of the plate 40,corresponding to the upper median line 37 of the principal template 7,is aligned over the upper circumferential median line 46 of the anvilsurface, corresponding to the upper median circumferential line 48 ofthe toroidal surface segments 35, 47. In this manner, the sides 48 ofthe plate 40 will lie substantially symmetrically astride the uppercircumferential median line 46 of the anvil 41.

To form a plate 40 that extends significantly on both sides of eitherthe first 10 or second 12 junctures of the principal template 7, it ispreferable to prepare an anvil 41 having joined upper surface segments35, 37 analogous to those depicted in FIG. 5. However, when a plate 40extends only slightly over one of these junctures 10, 12, it has beenfound satisfactory to substitute for the adjoining toroidal surface, ashort straight cylindrical surface, of the same circular diameter.

In FIG. 9, a series of individual plates 50 are shown, corresponding tosubtemplates 50A of the principal template 7.

The corners 51 on certain tapered thoracic plates 52 are shown asangled. This angling of the end 49 of such plates 52 is thoughtdesirable to minimize the amount of plate present, consistent with theclosely-packed spacing criteria preferred for the pattern of holes 53,as described further below. Where the hole pattern permits, plates 50may have square-cut ends 55.

In FIG. 7 the preferred hole pattern in a plate 56 which is shaped toextend to the lower-most lumbar vertebrae L-5 is shown.

The holes 53 have "forbidden zones" 54 which are defined by circles ofdouble the radius 57 for each hole 53 including the countersunk portions(not shown). The close-packing criteria requires that such forbiddenzones 54 not overlie another hole 53, nor the edges 59 of the plate 56.Otherwise, the holes 53 are distributed symmetrically about the medianline 60 of the plate 56 along which centrally located holes 61 areplaced, each hole 53 being as close to the central holes 61 as thisclose-packing criteria will permit.

As can be seen in FIG. 9, once the template narrows sufficiently, as insample sub-plate 62 shown in detail in FIG. 7a, the hole pattern can nolonger sustain a centrally placed hole 61 and achieve maximum packingdensity. Thereafter, the tapered portion holes 63, should be staggered,in pairs, again as closely packed as possible without violating theirforbidden zones 54.

The attachment format by which plates may be fastened to vertebrae isshown in FIG. 8. A vertebrae 64 is penetrated by two screws 65 havingspheroidal heads 66 and seats 66a. The inner surface 67 of the plate 68contacts the vertebrae 64 principally along the line of its outsideedges 69. But ideally, when drawn-up tightly, the inner surface 67 ofthe plate 68 just contacts or lies slightly above the bone of thevertebrae 64. Thus the curve provided to the inner surface 67 of theplate 68 maximizes the stance and stability with which it engages thevertebrae 64.

The screws 65 preferably penetrate entirely through the vertebrae 64,being selected to be of this precise length. While the angled hole 70below the spherical seat 66a is drilled at 7 degrees from the normal 71to the median line 72 on the plate 68, the loose fit of the screws 65and the spherical seat 66a allows the screws 65 to be inserted inparallel orientation. The selection of angle and length for the screwswill be a matter of choice for the surgeon, according to the exigenciesof the operation.

CONCLUSION

The foregoing has constituted a description of specific embodimentsshowing how the invention may be applied and put into use. Theseembodiments are only exemplary. The invention in its broadest, and morespecific aspects, is further described and defined in the claims whichnow follow.

We claim:
 1. A preformed spinal plate for fitting to a portion of thehuman spine, said plate comprising:(1) an upper surface and aspine-opposing lower surface, said surfaces being bounded by outer sidesurfaces extending between sand upper and lower surfaces; (2) said uppersurface defining an imaginary median line extending longitudinally forthe length of said plate and terminating at top and bottom end surfaces,said side surfaces being symmetrically disposed on either side of saidmedian line; (3) said lower surface being transversely concave incurvature about said longitudinal median line; and (4) said sidesurfaces being tapered so as to progressively narrow as proceeding inthe longitudinal direction defined by said median line towards saidupper end surface,wherein said median line is curved laterally withrespect to said side surfaces while lying within a single plane.
 2. Apreformed spinal plate as in claim 1 wherein said median line is curvedalong an arc of a circle lying within said plane.
 3. A preformed spinalplate as in claim 1 wherein said side surfaces are both curved alongpaths which, if extended would intersect.
 4. A preformed spinal plate asin claims 1, 2 or 3 wherein said spine-opposing lower surface is shapedto lie along the upper circumferential median line of a toroidal surfacesegment, said upper circumferential median line being the locus ofcontact between the top side of a torus and a plane surface overlyingsuch top side.
 5. A preformed spinal plate as in claim 4 comprising aplurality of circular countersunk holes formed therein, said holes:(1)being located in staggered pairs, one on each side of said longitudinalmedian line, and (2) all having an outside diameter and correspondingradii and being distributed along the plate in a close-packed patternwhereby said holes are separated from each other by at least three holeradii and from said side surfaces by at least two hole radii.
 6. Apreformed spinal plate as in claim 5 wherein said holes are separatedfrom each other by substantially three hole radii and from said sidesurfaces by substantially two hole radii.